Researchers at ETH Zurich (Eidgenössische Technische Hochschule Zürich), along with teams from Germany and Australia, have made some incredibly precise measurements using calcium atoms—and the results might hint at something beyond what current physics can explain. While they’re not claiming to have found new physics just yet, their work pushes the boundaries of what we know and tightens the limits on theories that go beyond the Standard Model.
The Standard Model is the best framework scientists have for understanding particles and forces, but it doesn’t cover everything like dark matter, which makes up most of the universe’s mass. Some theories suggest there could be a fifth fundamental force, possibly carried by a new particle, that interacts between electrons and neutrons. To test this idea, the researchers looked at tiny differences in atomic transitions called isotope shifts, in calcium ions.
They focused on two specific transitions: ³P₀ → ³P₁ in Ca¹⁴⁺ and ²S₁/₂ → ²D₅/₂ in Ca⁺. These were measured across five stable calcium isotopes (Ca⁴⁰, Ca⁴², Ca⁴⁴, Ca⁴⁶, and Ca⁴⁸), which all have 20 protons but different numbers of neutrons. Measuring these shifts with sub-Hertz precision is no small feat. “We trapped two isotopes at the same time in the ion trap and measured them together,” said doctoral student Luca Huber. This clever setup helped cancel out noise and allowed them to measure frequency differences down to 100 millihertz.
Meanwhile, other teams helped out by measuring transitions in highly charged calcium ions and calculating nuclear mass ratios with incredibly tiny uncertainties—less than 4 × 10⁻¹¹. When all this data was combined, the researchers created what’s called a King plot (KP). Normally, if everything follows known physics, the data points in a King plot should line up neatly. But in this case, they didn’t.
“The key thing about these King plots: if all the points lie on a straight line, the measured values can be explained by known nuclear physics effects,” said Diana Prado Lopes Aude Craik, a physics professor at ETH Zurich. But the data showed a clear curve, a nonlinearity, with a significance of about 10³ σ, which is way beyond what could be chalked up to random chance.
So what’s causing this bend in the plot? The team ran detailed calculations and found that the biggest known effect from the Standard Model, the second-order mass shift, wasn’t enough to explain it. The only remaining known factor that might be strong enough is something called nuclear polarization. That’s when the nucleus of an atom gets slightly distorted by the surrounding electrons. It’s not well understood, but it could be the missing piece.
Even though the results don’t confirm new physics, they do help tighten the limits on what’s possible. The team used their data to improve constraints on a hypothetical Yukawa interaction, a type of force that could be carried by a new boson. Their measurements narrowed down the possibilities for boson masses between 10 eV/c² and 10⁷ eV/c².
The researchers aren’t stopping here. They’re already working on measuring a third transition in calcium with even higher precision. “We hope that this will help us overcome the theoretical challenges and make further progress in the search for this new force,” said Aude Craik.
This is a great example of how cutting-edge measurement techniques like ultra-precise ion trapping and frequency analysis can push the limits of what we know about the universe. Even if you’re not deep into particle physics, it’s exciting to see how atomic-level experiments can help test big ideas like new forces and particles.
Source: ETH Zurich, American Physical Society
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